Hi,
I am trying to move subversion.but i am not able to understand below point:
"Branching and tagging are cheap (constant time) operations
There is no reason for these operations to be expensive, so they aren't.

Branches and tags are both implemented in terms of an underlying "copy" operation. A copy takes up a small, constant amount of space. Any copy is a tag; and if you start committing on a copy, then it's a branch as well. (This does away with CVS's "branch-point tagging", by removing the distinction that made branch-point tags necessary in the first place"